The formation of Subpolar Mode Water (SPMW) is documented by combining neodymium isotopic composition (Nd IC) as a conservative tracer with salinity, temperature and O2 concentration. Nd IC reveals a more pronounced eastward extension of the Labrador Current, particularly its Subarctic Intermediate Water component, than the other parameters. Nd IC also suggests that waters flowing through the Denmark Strait do not significantly contribute to the SPMW formation. A two end-member mixing, involving Subarctic Intermediate Water (and maybe ultimately Labrador Current Water) and North Atlantic Central Water, is proposed to explain SPMW formation. These results provide important insight into the processes controlling the Nd IC in the North Atlantic Subpolar Gyre, which could provide a means for reconstructing the northward extension of the Gulf Stream in the past.